The present invention relates to an image reading apparatus, such as a scanner, for optically reading an original by driving a scanning optical system using a pulse-controlled motor.
There is known an image reading apparatus, such as a scanner section of a copying machine or a scanner, for optically reading an original by driving a scanning optical system using a pulse-controlled motor, such as a stepping motor or an encoder-equipped DC servomotor, whose rotation amount or rotation speed can be controlled by a pulse number or a pulse cycle.
In the image reading apparatus for optically reading an original by driving a scanning optical system using a pulse-controlled motor, the pulse (stepping) motor is controlled at the time of reading/scanning in a microstep driving mode by increasing/decreasing a division number in accordance with an enlargement/reduction magnification of an image.
Jpn. Pat. Appln. KOKOKU Publication No. 6-81229, for instance, discloses that a pulse division number for microstep driving is made greater than that for one magnification at the time of image enlargement, and made less than that for one magnification at the time of image reduction. Specifically, the pulse-controlled motor is microstep-driven by increasing/decreasing the division number in accordance with the enlargement/reduction of image.
In the above-described pulse motor driving control, the division number is normally set to be successively increase as the reading magnification is varied from the enlargement side to the reduction side.
In the conventional image reading apparatus, a carriage (a moving member integrated with a mirror, a light source, etc.) which scans an original in units of a line in a main scan direction is moved in a sub-scan direction, thereby reading the entire original. The carriage of the image reading apparatus, which moves in the sub-scan direction, is accelerated by a motor up to a predetermined scanning speed in accordance with a scanning magnification, and the carriage is then moved at the predetermined scanning speed to read the original.
Although the carriage for scanning is driven by the stepping motor (pulse motor) to read the image, a torque variation occurs due to each pulse for driving the stepping motor. A vibration due to the torque variation may affect the carriage and degrade the quality of the read image.
The conventional stepping motor is driven by a half-step driving method or a microstep driving method, instead of a standard full-step driving method. Thereby, the frequency of drive pulses is increased and the amount of movement per pulse is decreased. Thus, the variation in torque is decreased, and the torque variation due to the motor is reduced. According to the adopted motor driving method, the frequency of drive pulses is highest in a range permitted by hardware and software.
However, where the rotation speed of the motor varies in accordance with the read magnification, the motor is driven with smaller drive steps at the time of low-speed rotation, and with larger drive steps at the time of high-speed rotation than at the time of the low-speed rotation, because of restrictions in hardware and software.
In the prior art and the above-described driving method, where the motor is driven with smaller step angles at the time of low-speed rotation than at the time of high-speed rotation (“microstep driving”), the vibration may increase in some cases by the microstep driving (i.e. by increasing the division number).
Furthermore, in some cases, the stepping motor may resonate at a certain frequency, increasing the vibration and noise and decreasing the torque.